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From:   Alexander Potapenko <glider@google.com>
Date:   Fri, 25 Mar 2022 14:15:13 +0100
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Subject: Re: [PATCH 16/43] kmsan: mm: call KMSAN hooks from SLUB code
To:     Vlastimil Babka <vbabka@suse.cz>
Cc:     Alexander Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andrey Konovalov <andreyknvl@google.com>,
        Andy Lutomirski <luto@kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Arnd Bergmann <arnd@arndb.de>, Borislav Petkov <bp@alien8.de>,
        Christoph Hellwig <hch@lst.de>,
        Christoph Lameter <cl@linux.com>,
        David Rientjes <rientjes@google.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Eric Dumazet <edumazet@google.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        Ilya Leoshkevich <iii@linux.ibm.com>,
        Ingo Molnar <mingo@redhat.com>, Jens Axboe <axboe@kernel.dk>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Kees Cook <keescook@chromium.org>,
        Marco Elver <elver@google.com>,
        Matthew Wilcox <willy@infradead.org>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Pekka Enberg <penberg@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Petr Mladek <pmladek@suse.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Vegard Nossum <vegard.nossum@oracle.com>,
        Linux Memory Management List <linux-mm@kvack.org>,
        Linux-Arch <linux-arch@vger.kernel.org>,
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> >  static inline void *get_freepointer(struct kmem_cache *s, void *object)
> >  {
> >       object = kasan_reset_tag(object);
> > -     return freelist_dereference(s, object + s->offset);
> > +     return kmsan_init(freelist_dereference(s, object + s->offset));
>
> ... but I don't see why it applies to get_freepointer() too? What am I missing?

Agreed, kmsan_init() is not needed here.

> >  }
> >
> >  static void prefetch_freepointer(const struct kmem_cache *s, void *object)
> > @@ -357,18 +361,28 @@ static void prefetch_freepointer(const struct kmem_cache *s, void *object)
> >       prefetchw(object + s->offset);
> >  }
> >
> > +/*
> > + * When running under KMSAN, get_freepointer_safe() may return an uninitialized
> > + * pointer value in the case the current thread loses the race for the next
> > + * memory chunk in the freelist. In that case this_cpu_cmpxchg_double() in
> > + * slab_alloc_node() will fail, so the uninitialized value won't be used, but
> > + * KMSAN will still check all arguments of cmpxchg because of imperfect
> > + * handling of inline assembly.
> > + * To work around this problem, use kmsan_init() to force initialize the
> > + * return value of get_freepointer_safe().
> > + */
> >  static inline void *get_freepointer_safe(struct kmem_cache *s, void *object)
> >  {
> >       unsigned long freepointer_addr;
> >       void *p;
> >
> >       if (!debug_pagealloc_enabled_static())
> > -             return get_freepointer(s, object);
> > +             return kmsan_init(get_freepointer(s, object));
>
> So here kmsan_init() is done twice?

Yeah, removing it from get_freepointer() does not introduce new
errors. I'll fix this in v2.